1. Technical Field
This invention relates generally to electronic circuits and in particular to an electronic circuit that provides an indication of when a supply voltage has reached a particular voltage level, such as on start-up or reset.
2. State of the Art
Electronic circuits often require an indication of when a supply voltage has reached a specific voltage level after power-up or reset. After circuitry is turned on or reset, some parts of an electronic system may be kept dormant until the supply voltage reaches a certain level in order to restart circuitry or systems in a controlled and known manner. It is common to use a Power-On Reset (POR) circuit to provide an indication of when a supply voltage has reached a certain level. In systems containing digital circuitry, POR circuits are used to provide a power-on reset signal indication, in the form of a voltage transition, of when the supply voltage reaches a certain predetermined voltage level. The signal indication is used to place digital logic in a known state when the supply voltage is initially applied to the system.
Issues can arise, however, related to the fact that the amount of time it takes the supply voltage to reach its final value can vary greatly depending on the application. The specifications for some electronic systems are defined relative to when a supply voltage settles to its final value. For example, oscillator start-up times may be defined as settling to within a certain value within a specific time after the supply voltage reaches its maximum voltage value. In these cases, it is desirable to have the supply voltage level that triggers the transition of the power-on reset signal to be just slightly below the supply voltage maximum value, so that the time interval between the supply voltage reaching the trigger value and the supply voltage reaching its maximum value is minimally affected by the rate at which the supply voltage rises to its maximum value. It is also desirable to have the voltage level of the trigger voltage be insensitive to circuit process and temperature variations. In addition, it is desirable for fluctuations and/or perturbations of the voltage level of the supply voltage to not cause the power-on reset signal to transition. These two requirements have been mutually exclusive in many POR circuits because setting the trigger voltage close to the maximum supply voltage value can result in false triggering of the power-on reset signal when the supply voltage fluctuates below the trigger voltage level. Thus it is desirable to have a POR circuit that provides an indication of when a supply voltage reaches a predetermined trigger voltage level, to have this trigger voltage level be insensitive to process and temperature variations, and to have the capability to set the trigger voltage level asymptotically close to the maximum value of the supply voltage.